This invention relates generally to pumping apparatus and to pump control apparatus and methods. The invention more particularly, but not by way of limitation, relates to an apparatus for operating a pump which includes a movable member whose position correlates with the fluid output of the pump. It also relates to a method of metering fluid at a selected flow rate with a pump which includes a fluid end member that is reciprocated through a pumping cycle in response to reciprocation of a power end member by a controlled volume of actuating fluid communicated to the power end member, which pumping cycle includes an extend phase during which the power end member displaces fluid across a first area and a retract phase during which the power end member displaces fluid across a second area.
In various industries different types of fluids need to be pumped at accurately controllable rates to produce desired blends of materials. In the oil and gas industry, for example, blends of carefully controlled amounts of fluids are made to produce fracturing fluids. A fracturing fluid is a mixture which is pumped down a well to fracture a subterranean formation to improve the flow of hydrocarbons from the formation into the well, thereby enhancing production of the well. A fracturing fluid contains one or more different additives such as buffers, crosslinkers, surfactants, clay stabilizers, friction reducers, liquid gel concentrates, liquid fluid loss materials and paraffin chemicals. These include organic acids (e.g., glacial acetic acid), petroleum-based fluids (e.g., diesel, kerosene), alcohols (e.g., methanol), solvents (e.g., xylene and toluene), mineral acids (hydrochloric acid), bases (e.g., sodium hydroxide) and amines. These liquid additives have different viscosities and typically need to be added in different quantities to produce a desired fracturing fluid. Therefore, they need to be metered by a pump at different metering or flow rates. By way of a specific example, the following are characteristics for the aforementioned chemicals in typical applications made by Halliburton Services, the Halliburton Company division which performs oil and gas well fracturing services:
______________________________________ Flow Max- Fluid Flow Rate Rate Viscosity imum Temperature Range Control Range Pressure Range (gal/min) Accuracy (cps at 70.degree. F.) (psi) (.degree.F.) ______________________________________ 0.05-52 .+-.2% 1-600 200 additive pour point to 120 ______________________________________
Metering pumps are used on fracturing equipment to accurately inject chemicals, such as those mentioned above, into the composite fracturing fluid. The design requirements for a general purpose fracturing service metering pump are very demanding because of the many types of chemicals pumped and the wide range of flow rates over which they are typically pumped. Currently, several different types of pumps are used to meter the different additives and to accommodate the different flow rates. These include gear pumps (e.g., ECO G-8), progressive cavity pumps (e.g., Roper 72292), peristaltic pumps (e.g., Waukesha SP-25) and single-acting simplex pumps (e.g., Milton-Roy MILROYAL C-4 7/16). These pumps typically have a limited maximum pressure capability (e.g., 50-200 psi), and limited flow rangeability or turn-down ratio (e.g., 5:1-100:1).
Although there are individual apparatus which can be used to pump certain ones of the additives at the desired flow rates in the specific example concerning the production of a fracturing fluid, there is no pumping apparatus currently available which will reliably meter, throughout a substantially encompassing flow rate range, all of the liquid additives identified above with acceptable accuracy. Therefore, there is the need for such an apparatus in at least the application of metering fracturing fluid additives. More specifically, there is the need for a pumping apparatus or a control apparatus or method for a pump which can satisfy substantially more of the specific pumping characteristics listed in the table above than an existing metering pump has been able to satisfy. It is further contemplated that such a versatile apparatus or method capable of pumping different fluids across a broad spectrum of viscosities and throughout a broad range of flow rates is needed in general. Such a pumping apparatus should also, of course, meet conventional pump characteristics such as pressure and temperature ranges at which the apparatus needs to be used. The fluid-end of such an apparatus should have good chemical compability and good suction characteristics for pumping additives from remote locations. The controls at the power end of such an apparatus should be adaptable for interfacing with other equipment and be packaged in a ruggedized enclosure, especially for the exemplary application in producing fracturing fluids.